bitmap: prevent invalid cell access
bitmap: prevent invalid cell access

Fixes segfaults when big brush goes out of bounds.
That was dumb... I wrote the tests, but they were
executed just after accessing memory...

war2/war2edit,jeanguyomarch/war2edit,war2/war2edit,jeanguyomarch/war2edit,jeanguyomarch/war2edit,war2/war2edit